Sportster Blow By Repair

A few thousands miles after my Harley Davidson dealer serviced my Sportster for a blow-by oil leak, oil began to puddle again along the right side engine block. Not ready to install a blow-by kit, I decided to try my own repair. One of the problems with the factory blow-by solution is that the blow-by vents are too far from the vacuum crated by the carburetor inlet, allowing most of the blow-by to puddle to the bottom of the Sportster air filter housing and leak onto the engine.


My solution is to route the blow-by oil over the top of the carburetor inlet. By doing this, more of the blow-by oil should be sucked into the Sportster carburetor. By routing hose from the molded tubes in the air filter assembly to a center point above the carburetor inlet, the blow-by will exit directly over the carburetor intake, and not off to the sides as designed by the factory.


Routed Vacuum Hoses and Tee

I used vacuum hose that was close to the inside diameter of the molded tubes in the air filter assembly. Fuel line would have been a better solution, but would not have been flexible enough to route above the Sportster carburetor inlet. The blow-by oil will eventually deteriorate the vacuum hose, and the hose will need to be replaced.

Route a section of hose through each blowby tube, and pull it though the air inlet opening as in the above picture. Once both sections of tube are routed, connect the vacuum tee between them, with the opening in the vacuum tee facing toward the carburetor inlet. Carefully pull the vacuum hose from both sides until the tee settles inside the air filter assembly.

Sportster blow by repair

Close-up of Vacuum Hose Tee

Cut the excess vacuum hose flush with the molded tubes on the air filter assembly. The blowby bolts that attach to the heads measure 1/2 inch in length. The molded tubes that they fit into on the air filter assembly measure 3/4 of an inch.

When the air filter assembly is re-installed, the blow-by bolts will push the vacuum hose forward into the molded tubes, leaving 1/4 of an inch of vacuum tube still in the molded tube to re-route the blow-by over the carburetor.

This Sportster blow-by solution cost less than $3.00 and took less than a half hour to install.